A liquid crystal display device is a display device in which a liquid crystal composition is used for display, and a representative display method thereof is controlling the amount of light transmitted through a liquid crystal panel in which the liquid crystal composition is sealed between a pair of substrates by irradiating the liquid crystal panel with light from a backlight and by applying voltage to the liquid crystal composition to change the alignment of liquid crystal molecules. Such a liquid crystal display device has merits such as being thin and light and consuming a small amount of electricity, and thus is used for electronic devices such as smartphones, tablet PCs, and car navigation systems. In recent years, the definition of pixels has increased for use of, for example, smartphones, and there has been a tendency that the number of conductive lines and the area of the black matrix provided in the liquid crystal panel increase along with this.
Typically, in a liquid crystal display device, the alignment of liquid crystal molecules in a state in which no voltage is applied is controlled by an alignment film that has been subjected to alignment treatment. Conventionally, as a method of alignment treatment, a rubbing method of rubbing the surface of the alignment film by, for example, a roller, has been widely used. However, since the number of conductive lines and the area of the black matrix provided in a liquid crystal panel have increased, steps have become more likely to be formed in the surface of a substrate in the liquid crystal panel. If steps are present in the surface of the substrate, the vicinity of the steps cannot be rubbed appropriately by using a rubbing method in some case. In the case where alignment treatment is not uniformly performed, decrease of a contrast ratio of the liquid crystal display device is caused.
To address this, in recent years, research and development have been promoted on a photo-alignment method of irradiating the surface of an alignment film with light as a method of alignment treatment that replaces the rubbing method. According to the photo-alignment method, alignment treatment can be performed without contact with the surface of the alignment film. Therefore, the alignment treatment is less likely to be uneven even in the case where steps are present in the surface of the substrate, and a merit that good liquid crystal alignment can be realized on the entire surface of the substrate can be achieved.
In addition, the increase of the number of conductive lines and the area of the black matrix provided in the liquid crystal panel decreases the area ratio (opening ratio) of an opening portion that can be used for displaying. The decrease of the aperture ratio directly leads to decrease of the amount of light that can be transmitted through the liquid crystal panel, and thus greatly improving the luminance of a backlight in order to retain the display performance of the liquid crystal display device in terms of, for example, a contrast ratio, has been considered.
Meanwhile, with regard to a liquid crystal composition used for a liquid crystal display device, it has been desired that the stability thereof is enhanced such that the liquid crystal composition endures a load during a production process of the liquid crystal display device and a produced liquid crystal display device can exhibit a stable characteristic for a long period of time. For example, Patent Literature 1 discloses adding a hindered amine photostabilizer to a liquid crystal composition to suppress decomposition of the liquid crystal composition caused by heating and exposure to ultraviolet light and improve the reliability of the liquid crystal display device particularly in terms of temporal change of a voltage holding ratio. In addition, Patent Literature 2 discloses that a liquid crystal composition that has a high voltage holding ratio and is stable against heat and light can be obtained by adding one, two, or more kinds of phenolic antioxidants to a liquid crystal compound having a negative anisotropy of dielectric constant.